fix OBS-studio DTLS failed (#557)

rtc-dtls/src/crypto/crypto_cbc.rs

@@ -8,12 +8,13 @@
 
 // https://github.com/RustCrypto/block-ciphers
 
-use aes::cipher::{block_padding::Pkcs7, BlockDecryptMut, BlockEncryptMut, KeyIvInit};
+use aes::cipher::{BlockDecryptMut, BlockEncryptMut, KeyIvInit};
 use p256::elliptic_curve::subtle::ConstantTimeEq;
 use rand::Rng;
 use std::io::Cursor;
 use std::ops::Not;
 
+use super::padding::DtlsPadding;
 use crate::content::*;
 use crate::prf::*;
 use crate::record_layer::record_layer_header::*;
@@ -70,7 +71,7 @@ impl CryptoCbc {
         rand::thread_rng().fill(iv.as_mut_slice());
 
         let write_cbc = Aes256CbcEnc::new_from_slices(&self.local_key, &iv)?;
-        let encrypted = write_cbc.encrypt_padded_vec_mut::<Pkcs7>(&payload);
+        let encrypted = write_cbc.encrypt_padded_vec_mut::<DtlsPadding>(&payload);
 
         // Prepend unencrypte header with encrypted payload
         let mut r = vec![];
@@ -101,7 +102,7 @@ impl CryptoCbc {
         let read_cbc = Aes256CbcDec::new_from_slices(&self.remote_key, iv)?;
 
         let decrypted = read_cbc
-            .decrypt_padded_vec_mut::<Pkcs7>(body)
+            .decrypt_padded_vec_mut::<DtlsPadding>(body)
             .map_err(|_| Error::ErrInvalidPacketLength)?;
 
         let recv_mac = &decrypted[decrypted.len() - Self::MAC_SIZE..];

rtc-dtls/src/crypto/mod.rs

@@ -4,6 +4,7 @@ mod crypto_test;
 pub mod crypto_cbc;
 pub mod crypto_ccm;
 pub mod crypto_gcm;
+pub mod padding;
 
 use std::convert::TryFrom;
 use std::sync::Arc;

rtc-dtls/src/crypto/padding.rs

@@ -0,0 +1,122 @@
+use cbc::cipher::block_padding::{PadType, RawPadding, UnpadError};
+use core::panic;
+
+pub enum DtlsPadding {}
+/// Reference: RFC5246, 6.2.3.2
+impl RawPadding for DtlsPadding {
+    const TYPE: PadType = PadType::Reversible;
+
+    fn raw_pad(block: &mut [u8], pos: usize) {
+        if pos >= block.len() {
+            panic!("`pos` is bigger or equal to block size");
+        }
+
+        let padding_length = block.len() - pos - 1;
+        if padding_length > 255 {
+            panic!("block size is too big for DTLS");
+        }
+
+        set(&mut block[pos..], padding_length as u8);
+    }
+
+    fn raw_unpad(data: &[u8]) -> Result<&[u8], UnpadError> {
+        let padding_length = data.last().copied().unwrap_or(1) as usize;
+        if padding_length + 1 > data.len() {
+            return Err(UnpadError);
+        }
+
+        let padding_begin = data.len() - padding_length - 1;
+
+        if data[padding_begin..data.len() - 1]
+            .iter()
+            .any(|&byte| byte as usize != padding_length)
+        {
+            return Err(UnpadError);
+        }
+
+        Ok(&data[0..padding_begin])
+    }
+}
+
+/// Sets all bytes in `dst` equal to `value`
+#[inline(always)]
+fn set(dst: &mut [u8], value: u8) {
+    // SAFETY: we overwrite valid memory behind `dst`
+    // note: loop is not used here because it produces
+    // unnecessary branch which tests for zero-length slices
+    unsafe {
+        core::ptr::write_bytes(dst.as_mut_ptr(), value, dst.len());
+    }
+}
+
+#[cfg(test)]
+pub mod tests {
+    use rand::Rng;
+
+    use super::*;
+
+    #[test]
+    fn padding_length_is_amount_of_bytes_excluding_the_padding_length_itself() -> Result<(), ()> {
+        for original_length in 0..128 {
+            for padding_length in 0..(256 - original_length) {
+                let mut block = vec![0; original_length + padding_length + 1];
+                rand::thread_rng().fill(&mut block[0..original_length]);
+                let original = block[0..original_length].to_vec();
+                DtlsPadding::raw_pad(&mut block, original_length);
+
+                for byte in block[original_length..].iter() {
+                    assert_eq!(*byte as usize, padding_length);
+                }
+                assert_eq!(block[0..original_length], original);
+            }
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    #[should_panic]
+    fn full_block_is_padding_error() {
+        for original_length in 0..256 {
+            let mut block = vec![0; original_length];
+            DtlsPadding::raw_pad(&mut block, original_length);
+        }
+    }
+
+    #[test]
+    #[should_panic]
+    fn padding_length_bigger_than_255_is_a_pad_error() {
+        let padding_length = 256;
+        for original_length in 0..128 {
+            let mut block = vec![0; original_length + padding_length + 1];
+            DtlsPadding::raw_pad(&mut block, original_length);
+        }
+    }
+
+    #[test]
+    fn empty_block_is_unpadding_error() {
+        let r = DtlsPadding::raw_unpad(&[]);
+        assert!(r.is_err());
+    }
+
+    #[test]
+    fn padding_too_big_for_block_is_unpadding_error() {
+        let r = DtlsPadding::raw_unpad(&[1]);
+        assert!(r.is_err());
+    }
+
+    #[test]
+    fn one_of_the_padding_bytes_with_value_different_than_padding_length_is_unpadding_error() {
+        for padding_length in 0..16 {
+            for invalid_byte in 0..padding_length {
+                let mut block = vec![0; padding_length + 1];
+                DtlsPadding::raw_pad(&mut block, 0);
+
+                assert_eq!(DtlsPadding::raw_unpad(&block).ok(), Some(&[][..]));
+                block[invalid_byte] = (padding_length - 1) as u8;
+                let r = DtlsPadding::raw_unpad(&block);
+                assert!(r.is_err());
+            }
+        }
+    }
+}